1. Field of the Invention:
The invention relates to load selectors for switching over from one tap of a step transformer to the next, including fixed step contacts being connected to the taps and being disposed on a circle, at least two switches per phase being simultaneously pivotable on the circle and disposed on a common contact carrier, two vacuum switch contacts per phase being electrically built into the pivotable contact carrier and connected in series with the switch contacts, and a cam race disposed on the circle for controlling rollers for actuating the vacuum switches contacts.
Load selectors, like step switching devices that have separate load reversers and selectors, serve the purpose of switching over taps of control windings of transformers under load in order to purposefully compensate for consumer voltage changes. Load selectors become less expensive to manufacture and use than the step switching devices predominantly used in the past, by dispensing with the separation of the load reverser from the selector.
2. Description of the Related Art:
Even in the load selector, arcs occur at various contacts in the reversal process, and they not only destroy the transformer oil bathing them but they also contaminate the transformer oil with erosion products of the switch contacts, and therefore impair the electrical strength of the oil. In order to avoid such arcs, German Published, Prosecuted application No. DE-AS 27 31 133 B1 suggests switching the step short-circuit current on and off at vacuum switches. To this end, in the known configuration, one vacuum switch is connected in series with each switch contact roller, and another vacuum switch is also connected in series with a transition resistor per contact carrier. All that is taught by that document about the structure of a load selector of that kind having vacuum switches, is that the vacuum switches are accommodated on the pivotable contact carrier, and that rollers provided for controlling it carry the load current.
Published European application No. 0 151 740 discloses the structural layout of a load selector without vacuum switches. In that construction, three contact rollers per phase are provided. The rollers run on fixed counterpart contacts simultaneously acting as switch cams and they control the movements of parts of the pivotable contact carrier. When pivoting the contact carrier, a step short-circuit current limited by a transition resistor is first switched on, and interrupted again immediately thereafter as the contact carrier pivots onward. The effect of the arcs created between the fixed switch contacts and the contact rollers in such a case is then as discussed initially above.